The overall objective of this research is to investigate the movements of normal tissue cells and cancer cells, both in culture and in the organism, in order to increase our understanding in general of cell locomotion in multicellular organisms and, in particular, of the invasive movements of embryonic cells in the construction of tissues and organs and of cancer cells during malignancy. In our recent pursuit of this objective, we have studied: 1) the sources of cell surface membrane during cell spreading and in the formation of locomotory protuberances from the cell surface; 2) the role of cytoplasmic microfilaments and microtubules in cell locomotion, both in vitro and in vivo; 3) the structure of the plasma membrane as related to cell surface adhesiveness; 4) the contact relations of cancer cells with each other and with normal cells, particularly as regards their contact inhibitory behavior; and 5) the changes in cell properties that lie at the basis of the change to the neoplastic state, in particular changes in the cyto- skeleton and cell surface adhesiveness.